1. Field of the Invention
This invention relates generally to linear aromatic polyimides. It relates particularly to maximizing the level of crystallinity in linear aromatic polyimides without degrading the molecular weight thereof.
2. Prior Art
Linear aromatic polyimides are well known to have exceptional thermal and oxidative stabilities. However, they are very difficult to process into useful forms, because of their poor softening/flow properties. Most of these polymers have to be processed at temperatures approaching 400.degree. C. and at pressures of 500 to 10,000 psi. At these high temperatures and pressures a sintering of finely divided polymer particles occurs. In order for linear aromatic polyimides to be useful as adhesives, molding powders and matrix resins for various applications, the polymers should exhibit flow at lower temperatures and pressures.
Accordingly, a primary object of the present invention is to provide a process for converting the polyamide-acid precursors of polyimides to polyimides which exhibit maximum levels of crystallinity, so that these crystalline forms can then be melted at relatively moderate temperatures to afford adequate flow for molding and consolidation operations.
Another object of the present invention is to provide a method for maximizing and controlling the level of crystallinity that develops in polyimides as they are chemically cyclodehydrated from their corresponding polyamide-acid precursors.
A further object of the present invention is to provide a process for chemically converting polyamide-acids to the corresponding semi-crystalline polyimides without degrading the molecular weight of the polyimides.